1. Field of the Invention
The invention relates to the manufacture of hot-rolled and cold-rolled sheet from iron-carbon-manganese austenitic steels having very high mechanical properties, and especially a high mechanical strength combined with excellent resistance to delayed cracking.
2. Description of the Related Art
It is known that certain applications, especially in the automotive field, require metal structures to be further lightened and strengthened in the event of an impact, and also good drawability. This requires the use of structural materials that combine a high tensile strength with great deformability. To meet these requirements, patent FR 2 829 775 discloses for example austenitic alloys, having as main elements iron/carbon (up to 2%) and manganese (between 10 and 40%), which can be hot-rolled or cold-rolled and have a strength that may exceed 1200 MPa. The mode of deformation of these steels depends only on the stacking fault energy—for a sufficiently high stacking fault energy, an observed mode of mechanical deformation is by twinning, which results in a high work hardenability. By acting as an obstacle to the propagation of dislocations, the twins help to increase the yield strength. However, when the stacking fault energy exceeds a certain limit, perfect dislocation slip becomes the dominant deformation mechanism and the work hardenability is reduced. The aforementioned patent therefore discloses grades of Fe—C—Mn steel whose stacking fault energy is such that a high work hardenability is observed, combined with a very high mechanical strength.
Now, it is known that the sensitivity to delayed cracking increases with the mechanical strength, in particular after certain cold-forming operations since high residual stresses are liable to remain after deformation. In combination with atomic hydrogen possibly present in the metal, these stresses are liable to result in delayed cracking, that is to say cracking that occurs a certain time after the deformation itself. Hydrogen may progressively build up by diffusion into the crystal lattice defects, such as the matrix/inclusion interfaces, twin boundaries and grain boundaries. It is in the latter defects that hydrogen may become harmful when it reaches a critical concentration after a certain time. This delay results from the residual stress distribution field and from the kinetics of hydrogen diffusion, the hydrogen diffusion coefficient at room temperature being low, more particularly in austenitic structural alloys in which the mean path per second of this element is around 0.03 microns. In addition, hydrogen localized at the grain boundaries weakens their cohesion and favors the appearance of delayed intergranular cracks.
There therefore exists a need to have hot-rolled or cold-rolled steels that exhibit simultaneously a high strength and a high ductility, combined with a very high resistance to delayed fracture.
There is also a need to provide such steels inexpensively, that is to say under manufacturing conditions compatible with the productivity requirements of existing industrial lines, and with acceptable costs for this type of product. It is known in particular that it is possible to significantly reduce the hydrogen content by specific degassing heat treatments. Apart from the additional cost of these treatments, their thermal conditions possibly result in grain coarsening or in cementite precipitation in these steels, often incompatible with the requirements in terms of mechanical properties.
The object of the invention is therefore to provide a hot-rolled or cold-rolled steel sheet or product that is inexpensive to manufacture, has a strength of greater than 900 MPa, an elongation at break of greater than 50%, is particularly suitable for cold forming and has a very high resistance to delayed cracking, without any particular need for a specific degassing heat treatment.